The present invention generally relates to spectroscopy and in particular relates to spectrometers and methods of spectroscopy for the energy analysis of charged particles.
Charged particle spectroscopy is a powerful tool in space science. The energy analysis of the ambient charged particles in outer space provides an understanding of geophysical and extraterrestrial phenomena. Charged particle spectroscopy in space generally involves energy analyzing the charged particles that flow from various directions toward the spacecraft. The spectra collected help us understand atmospheric phenomena such as solar photoionization of the earth""s upper atmosphere and extraterrestrial phenomena such as changes in the solar wind over the solar cycle. The knowledge gained from such instruments also helps us model conditions in outer space.
Since the flow of charged particles in outer space is generally low, it is of great importance to fly instruments with a large geometric factor in order to collect data as quickly as possible. The geometric factor is proportional to the product of the charged particle energy analyzer""s entrance aperture area and its solid angle of acceptance. The sensitivity of the instrument (the rate at which particles are counted for a given ambient particle flux) is proportional to the instrument""s geometric factor.
In general, there is an inverse relationship between geometric factor and energy resolution for electrostatic energy analyzers. In practice, slit width is often narrowed to increase energy resolution. By narrowing slit width, geometric factor and sensitivity are reduced due to the decreased area of the entrance aperture. High energy resolution instruments tend to have a low geometric factor and high geometric factor instruments tend to have low energy resolution.
The trend in space science has been to sacrifice energy resolution in favor of geometric factor to compensate for the low particle fluxes in outer space. High geometric factor instruments can energy analyze the ambient charge particles very rapidlyxe2x80x94but at relatively low energy resolution. Spectrometers of inherently large geometric factor and low energy resolution now dominate the field, such as those classified as quadraspherical in design. Some details of the quadraspherical (quarter of a sphere), or xe2x80x9ctop hatxe2x80x9d, design instruments are described by C. W. Carlson et al. in Measurement Techniques in Space Plasmas: Particles, pp. 125-140, 1998.
Although the trend now is to fly compact, large geometric factor, quadraspherical charged particle analyzers, hemispherical electrostatic analyzers have flown in the past to provide very high energy-resolution spectra. Hemispherical electrostatic analyzers are preferred for high energy-resolution work because of their high charged-particle-optical efficiency and their lack of charged-particle-optical aberrations. One such instruments is described by Doering et al. in Radio Science, Vol. 8, No. 4, 1973, pp. 387-392, flew on three satellites in the 1970""s. The energy resolution of the instrument was 2.5% (change in energy divided by energy, full peak width at half maximum peak height). Charge particle analyzers now used for space flight rarely have energy resolution of better than 5%, and more commonly have energy resolution in the double digits.
There is now interest in collecting high energy-resolution spectra of charged particles in outer space. For instance, the determination of spacecraft floating potential is possible through an analysis of high energy-resolution electron energy spectra, as described in L. Goembel and J. Doering, Journal of Spacecraft and Rockets, Vol. 35, No. 1, pp. 66-72, 1998. It is important to measure spacecraft charge because even minor spacecraft charging biases scientific instruments (such as plasma spectrometers) and makes it difficult to interpret valuable data. In extreme cases rapid discharge from a spacecraft can cause costly system failures. Monitoring the charge and reducing it through a controlled discharge can prevent such damage. Other uses for high energy resolution electron spectra exist, such as in the determination of the ratio of ambient atomic oxygen to nitrogen in the upper atmosphere, as described by L. Goembel and J. P. Doering in Journal of Geophysical Research, Vol. 102, No. A4, pp. 7411-7419, 1997.
To date, there have been no compact, large geometric factor instruments capable of collecting high energy-resolution charged particle spectra in outer space. The high energy-resolution hemispherical analyzer-based instrument described by Doering et al. in Radio Science, Vol. 8, No. 4, pp. 387-392, 1973 would be considered bulky by today""s standards. It would also be considered slow to collect spectra by today""s standards since its geometric factor was small compared to the quadraspherical spectrometers that are currently in use. Designers of charged particle spectrometers appear to have reached an impasse in efforts to design a compact, high geometric factor, high-energy resolution instrument. Although the fully focusing charged particle optics of the hemispherical condenser design make it the preferred configuration for high energy-resolution spectroscopy, the large hemisphere that would be needed to collect data quickly with a spectrometer of the traditional design rules out the deployment of such an instrument. The accepted rule in the design of space flight charged particle spectrometers has been xe2x80x9cif sensor optics are focusing then little can be done to improve performance short of increasing sensor dimensionsxe2x80x9d, as quoted from D. T. Young, xe2x80x9cSpace Plasma Particle Instrumentation and the New Paradigm: Faster, Cheaper, Betterxe2x80x9d, p.8, Measurement Techniques in Space Plasmas: Particles, R. T. Pfaff, J. E. Borovsky, David T. Young, Editors, (Geophysical Monograph; 102), American Geophysical Union (Washington, D.C. 1998).
Much development of hemispherical charged particle energy analyzers has been done in fields outside of space science. The double-focusing property of the hemispherical analyzer has long been utilized in the field of surface imaging electron spectroscopy (XPS or ESCA). Hemispherical analyzers with extended arcuate slits such as shown in FIG. 6 of U.S. Pat. No. 3,733,483 to Green et al. (1973), FIG. 4a of U.S. Pat. No. 5,285,066 to Sekine et al. (1994), and FIG. 1 of U.S. Pat. No. 6,104,029 to Coxon et al. (2000) have been used to maximize the sensitivity of such instruments. In such imaging spectroscopy, focusing multi-element fore-optics are used to transmit an electron-spectroscopic image of the surface to the entrance plane of the hemispherical analyzer. The resulting image on the detector has one direction representing energy, and the perpendicular direction representing position on the original surface, as described by U. Gelius et al. in J. of Electron Spectroscopy and Related Phenomena Vol. 52, 1990, p. 761.
Traditional hemispherical charged particle analyzers for space flight have contained a circular entrance aperture, such as that of Doering et al. in Radio Science, Vol. 8, No. 4, pp. 387-392, 1973.
The present invention utilizes an arcuate entrance slit on a charged particle analyzer to retain energy resolution while increasing aperture area, and, thus, geometric factor. Unlike imaging spectrometers that have contained arcuate slits, the present invention does not utilize imaging fore-optics but has an arcuate collimator that defines the solid angle of acceptance of the instrument. The present invention maximizes the solid angle of acceptance of the instrument and maximizes the aperture area of the instrument so that the ambient charged particles can be collected with greatest efficiency. The double focusing property of the hemispherical analyzer is used to maximize the solid angle of acceptance and charged-particle-optical filling of the space between the hemispherical electrodes while retaining the superb energy resolution of the hemispherical design.
The present invention breaks through the perceived impasse in efforts to design a compact high energy-resolution, high geometric-factor charged particle analyzer. The present invention retains the energy resolution of instruments that have flown in the past, but vastly increases geometric factor, by using an arcuate slit for both the collimator and entrance aperture. It is possible to increase the geometric factor by nearly two orders of magnitude over the instrument in Doering et al. with no increase in instrument size. Such a dramatic increase in the geometric factor of the instrument with no increase in bulk makes the instrument of the present invention competitive with similarly sized space science instruments of quadraspheric or other lower resolution design. This invention makes it possible to collect the quality data needed to determine, for example spacecraft floating potential, with a compact instrument and with high temporal resolution.
The present invention provides a charged particle spectrometer with a large geometric factor and high energy resolution that is capable of obtaining charged particle spectra of the environment under investigation in a relatively short period of time.
The above object is achieved by a charged particle spectrometer containing a coaxial hemispherical charged particle energy analyzer having an input slit extending in the direction perpendicular to a radial direction of the hemispherical electrodes included in the energy analyzer, an input collimator for defining the field of view of the spectrometer which is also extending in the direction perpendicular to a radial direction of the hemispherical electrodes included in the energy analyzer and a detector placed at the output end of the hemispherical analyzer that is capable of detecting the charged particles that pass through the hemispherical analyzer.
Other objects and features of the invention will become obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.